The present invention relates to transflective liquid crystal display devices and more specifically to improvement of such devices for achieving enhanced display quality.
Liquid crystal display devices have become widely spread in recent years because of their advantages such as the ability to make thinner devices and offer lower power consumption than display devices that utilize cathode ray tubes.
Liquid crystal display devices are generally divided into two types: a transmissive type and a reflective type.
The transmissive liquid crystal display device utilizes light irradiated from a so-called backlight to provide a display and is widely used as the display for word processors, notebook personal computers and the like. When the transmissive liquid crystal display device is utilized in environments where the intensity of incident light on the device is high, such as the outdoors, it is difficult to observe a normal display.
The reflective liquid crystal display device reflects outside light and utilizes the light to provide a display and thus does not have a backlight, which, in turn, offers lower power consumption than the transmissive type. Accordingly, with the rapid spread of portable equipment, the reflective liquid crystal display device is receiving wide attention as the display for such equipment. The reflective liquid crystal display device is capable of providing a satisfactory display in environments with strong outside light, such as the outdoors, but incapable of providing a normal display in environments where the intensity of incident light on the device is low, such as nighttime.
Hence, a so-called transflective liquid crystal display device having the functions of both the transmissive and reflective liquid crystal display devices is receiving attention. For example, Japanese Unexamined Patent Publication No. 7-318929 suggests a liquid crystal display device that utilizes a substrate having a transflective reflecting film on the rear side thereof. In addition, Japanese Unexamined Patent Publication No. 11-109417 suggests a liquid crystal display device having pixels which include both transmissive and reflective electrodes.
The transflective liquid crystal display device exhibits excellent visibility regardless of the brightness of the environment in which it is used, but on the other hand, the device has lower brightness than the transmissive and reflective liquid crystal display devices and is lack of the colorfulness of the image. For example, in a transflective liquid crystal display device having both transmissive and reflective electrodes, by increasing the ratio of the area occupied by the reflective electrodes and increasing the intensity of the backlight, the display brightness can be increased both in a reflective display mode and in a transmissive display mode. These measures, however, contribute to an increase in the amount of power consumption in the transmissive display mode, resulting in the loss of the advantage of low power consumption provided by the liquid crystal display device.
Furthermore, as is suggested in the above publication, in a display mode such that the backlight is driven all the time and a reflective display is complemented by a transmissive display, an excellent image display can be realized even in bright environments but a power consumption equal to or higher than that of the transmissive liquid crystal display device is required. That is, this leads to the loss of the advantage of low power consumption provided by the reflective display mode.
While in the transmissive display mode, light irradiated from the backlight is transmitted through the liquid crystal layer only once, in the reflective display mode, incident light is transmitted though the liquid crystal layer twice, before and after reflecting at a reflecting means such as reflective electrodes. There is a demand to reduce variations in display quality due to the optical path difference between the two modes. Hence, Japanese Unexamined Patent Publication No. 11-242226 suggests a transflective liquid crystal display device in which the alignment of liquid crystal molecules in reflective display regions is different from the alignment of liquid crystal molecules in transmissive display regions. However, when, as is suggested in the above-described publication, a plurality of regions having liquid crystal molecules whose alignments vary from region to region are provided, the alignment of the liquid crystal molecules becomes discontinuous at the boundaries between the regions, resulting in formation of so-called disclination lines. The liquid crystal molecules in those regions suffer from alignment defects, ending up not contributing at all to a normal display or requiring a long period of time to obtain an intended alignment.
Moreover, as was described above, in the transmissive display mode, light irradiated from the backlight is transmitted through the color filter only once, and in the reflective display mode, incident light is transmitted through the color filter twice, before and after reflecting at a reflecting means such as reflective electrodes, and thus variations in the hue of display occur between the two modes. The above-described publication also suggests that the device provides a color display only in transmissive display portions and in reflective display portions a non-color display is provided. Specifically, color filters are provided in the transmissive display regions, and light in the reflective display portions only contributes to the pixel lightness. This technique, however, has difficulty providing a display with higher brightness because the brightness of the pixels is controlled only by the area of the transmissive display portions.
Hence, there has been a demand for transflective liquid crystal display devices capable of providing a display with higher image quality while maintaining the advantage of power-saving features provided by the liquid crystal display device.
For the liquid crystal display device, generally, an improvement in the display quality of the moving images, i.e., in response time, and an increase in viewing angle are demanded. Thus, optically compensated bend (OCB) mode liquid crystal display devices having excellent response time and viewing angle are receiving attention. In the OCB mode liquid crystal display panel, when a voltage is not applied between a pixel electrode 103 on an array substrate 102 and a counter electrode 106 on a counter substrate 105, liquid crystal molecules 100 exhibit a splay alignment as shown in FIG. 13a, and when a voltage is applied, the liquid crystal molecules exhibit a bend alignment as shown in FIG. 13b. In addition, as the driving mode for the reflective liquid crystal display device, a reflective OCB (Rxe2x80x94OCB) mode is suggested. As is shown in FIG. 14, in the Rxe2x80x94OCB mode, a liquid crystal molecule on the side of one of the electrodes exhibits a hybrid alignment such that the long axis of the liquid crystal molecule is oriented perpendicular to a surface of the reflective electrode, while a liquid crystal molecule on the side of the other electrode exhibits a bend alignment.
In the transmissive liquid crystal display device, the so-called field-sequential technique, which eliminates the need to use color filters, is widely studied. For example, Japanese Unexamined Patent Publication No. 9-101497 suggests a TN mode liquid crystal display device with a backlight made up of three color tubes of R (red), G (green), and B (blue) in which each of the R, G, and B tubes is sequentially turned on at regular intervals.
It is an object of the present invention to provide a liquid crystal display device having excellent display quality and capable of precisely controlling the alignment of liquid crystal molecules and of providing a display with high brightness and high color purity both in a transmissive display mode and in a reflective display mode.
According to the present invention, there is provided a transflective liquid crystal display panel including: a pair of substrates; a liquid crystal layer sandwiched between the substrates; pixel electrodes disposed on a surface of one of the substrates facing the liquid crystal layer; a counter electrode disposed on a surface of the other substrate facing the liquid crystal layer; and an alignment film covering the surface of each of the substrates facing the liquid crystal layer, wherein the pixel electrodes, each including an electrode for reflective display and an electrode for transmissive display, are disposed such that the distances to the other substrate from the electrode for reflective display and the electrode for transmissive display are different; and wherein liquid crystal molecules at the liquid crystal layer surface facing the electrode for reflective display in a region above the electrode for reflective display (reflective display region) are aligned in the same direction as liquid crystal molecules in a region above the electrode for transmissive display (transmissive display region) that are in the same plane as the molecules in the reflective display region, the plane being parallel to the principal surfaces of the substrates.
When the alignment of the liquid crystal molecules facing the electrode for reflective display is the same as the alignment of the liquid crystal molecules in the transmissive display region that are located in the same plane as the molecules facing the electrode for reflective display, it is possible to prevent formation of a liquid crystal boundary where the molecular alignment is discontinuous, which in turn prevents formation of disclination lines, and the diving of the liquid crystal molecules can be controlled with good response time, making it possible to obtain a liquid crystal display device with excellent display quality of moving images.
The present invention can be applied to liquid crystal display devices of various driving modes such as a twisted nematic (TN) mode and an optically compensated bend (OCB) mode.
For example, the liquid crystal layer in the transmissive display region is made thicker than the liquid crystal layer in the reflective display region, and the transmissive display region and the reflective display region are driven in the OCB mode and the Rxe2x80x94OCB mode, respectively. With this combination, the alignment of the liquid crystal molecules between the two regions can be made substantially uniform and also the variations in the hue of the pixel display between the two display modes can be minimized.
Generally, the electrode for reflective display and the electrode for transmissive display are formed in different layers on the same substrate, and therefore the thickness of the liquid crystal layer in a region where the electrode for reflective display is disposed is different from that of the liquid crystal layer in a region where the electrode for transmissive display is disposed. Hence, in order to prevent formation of disclination lines, it is preferable that the alignment films disposed in such regions be treated so that the alignment of the liquid crystal molecules in contact therewith is different in each region.
A plurality of regions having different alignment directions can be easily formed by using a so-called photo-alignment film. Specifically, by irradiating a photo-curing monomer or prepolymer film with ultraviolet light using a mask, regions having desired alignment directions can be formed. By performing back exposure using the reflective portion as a mask, a multi-domain liquid crystal layer can be obtained in a self-aligned manner. In addition, a plurality of similar regions can also be formed by a rubbing process using a mask.
In the liquid crystal display device of the TN mode, a spontaneous twist in the alignment of liquid crystal molecules can be applied which is induced by addition of a chiral material to the liquid crystal layer. When an alignment film provided with a uniform alignment treatment is formed on a surface of one of the substrates, preferably the counter substrate having a flatter surface, in contact with the liquid crystal layer, liquid crystal molecules in contact with the alignment film spontaneously exhibit a desired alignment without the need to provide an alignment treatment on a surface of the other substrate.
Moreover, the liquid crystal alignment film allows liquid crystal molecules to transition from a vertical alignment to a horizontal alignment by light irradiation, and thus a panel in which the transmissive portion is in the OCB mode and the reflective portion is in the Rxe2x80x94OCB mode can be easily obtained.
Here, it is possible to allow a display in the reflective display region to be normally black and a display mode in the transmissive display region to be normally white. In addition, when a structure for promoting easier transition from a splay alignment to a bend alignment upon driving is provided in the transmissive display region, alignment defects can be further reduced. Such a structure includes protrusions of various shapes. Since the protruding portions are weak in terms of the ability to control alignment, the alignment of liquid crystal molecules easily becomes unstable and thus the above-described alignment transition can be promoted. It should be noted that the alignment of liquid crystal molecules can be more effectively transitioned from a splay alignment to a bend alignment by providing regions having different alignment directions locally in the alignment film.
When the electrode for transmissive display is disposed lower than the electrode for reflective display and further a color filter layer is formed so as to cover the electrodes, the color filter layer in the reflective display region can be made thinner than that in the transmissive display region. For example, the thickness of the color filter in the transmissive display is made twice the thickness of the color filter in the reflective display region. By disposing the filter having different thicknesses in the transmissive display region and the reflective display region, the hue variations between the two display modes due to the optical path difference in the color filter layer can be corrected and thus the color reproducibility is significantly improved.
It is preferable that protrusions be provided in the reflective display region for scattering incident light and increasing the viewing angle. In addition, in the case where the pixel electrodes are disposed on an uneven surface, when the electrode for transmissive display is disposed in a flat region, where the scattering function is low and a contribution to an increase in the viewing angle is small, and the electrode for reflective display is disposed above the protrusions, a high scattering performance and a high transmittance can be obtained.
The electrode for reflective display is preferably formed in the upper layer, as was described above, to minimize the optical path difference. When the electrode for reflective display is disposed in the layer upper than the electrode for transmissive display and than switching elements such as thin film transistors so as to cover the switching elements, a displayable region can also be secured even on the switching elements, making it possible to provide a high brightness display.
Furthermore, according to the present invention, by employing the so-called field-sequential technique for transmissive display, a display with high brightness and good image quality can be realized both in the transmissive display mode and in the reflective display mode.
For example, there is provided a transflective liquid crystal display panel including: a pair of substrates; a liquid crystal layer sandwiched between the substrates; pixel electrodes disposed on a surface of one of the substrates facing the liquid crystal layer; a counter electrode disposed on a surface of the other substrate facing the liquid crystal layer; an alignment film covering the surface of each of the substrates facing the liquid crystal layer; and a light source, wherein color filters are disposed so as to oppose to the pixel electrode for reflective display and wherein while in a reflective display mode a color display is realized by coloring light using the color filters as is the case with conventional display devices, in a region corresponding to the electrode for transmissive display color filters are not disposed and thus light is colored by other means in a transmissive display mode. Although in the region corresponding to the electrode for transmissive display the color filters or other alternatives thereto are not disposed, a non-color layer is disposed instead of the color filters.
That is to say, in the transmissive display mode, a color display can be realized using a color time division light source instead of the color filter. By employing a field sequential technique, a high brightness display can be realized in the transmissive display mode. With the field sequential technique which does not require color filters, there is no need to concern about a reduction in intensity due to reflection and the like caused by the color filter. Thus, a display with higher brightness can be obtained than the case with the color filter, without increasing the intensity of the backlight, i.e., without increasing the power consumption. In addition, it becomes possible to display the pixels with an arbitrary RGB color. Consequently, the ratio of the area occupied by the electrode for reflective display can be increased and thus a high brightness display can be realized in the reflective display mode as well as in the transmissive display mode. Thereby, it is possible to obtain a liquid crystal display device capable of displaying excellent images with low power consumption, regardless of the brightness of the surrounding environments and the like.
For the color filter layer, color-variable color filters whose colors are changed by an external input can also be used in addition to common monochromatic filters. For example, by using a cholesteric liquid crystal, light of specified wavelengths is reflected. When the color-variable color filters are driven on a time division basis in accordance with the light source, the pixels can display an arbitrary RGB color and thus the brightness is increased. In addition, when the color of the color filter layer in the reflective display region is changed in accordance with the color of light irradiated from the light source, in the transmissive display mode, even if outside light enters, color mixing does not occur between the reflective display region and the transmissive display region, and thus high color reproducibility is achieved. In particular, when the color of the color filter layer is changed so that the peak wavelength of light emitted from the color time division light source substantially corresponds to a wavelength at which the color filter exhibits its peak transmittance, the color purity is the same in the reflective display region and the transmissive display region, and thus an excellent display can be obtained.
Generally, color filter layers used for the reflective panel have a transmittance of as high as approximately 70%. Thus, even light, which does not have corresponding wavelengths, transmits through individual RGB pixels. When a color filter layer, similar to that used for reflective display, is disposed in the transmissive display region and monochromatic light irradiated from the color time division light source is allowed to transmit through the color filter layer, it is also possible to obtain high brightness.
For the light source, a light emitting diode (LED) or an electroluminescent device which exhibits emission line peaks with small half widths is preferably used.
Furthermore, a transflective liquid crystal display device with high brightness is realized without employing a field-sequential technique.
There is provided a transflective liquid crystal display device including: a pair of substrates; a liquid crystal layer sandwiched between the substrates; pixel electrodes disposed on a surface of one of the substrates facing the liquid crystal layer, each of the pixel electrodes including an electrode for reflective display and an electrode for transmissive display; a counter electrode disposed on a surface of the other substrate facing the liquid crystal layer; an alignment film covering the surface of each of the substrates facing the liquid crystal layer; a color filter layer disposed so as to oppose to the pixel electrodes; and a light source for irradiating the liquid crystal layer with light through the electrode for transmissive display, wherein the light source irradiates, for example, white light having emission line peaks of R, G, and B which substantially correspond to wavelengths at which transmittance of the color filter layer exhibits a peak. When the peak value of the emission wavelength of the light source and the peak value of the transmission wavelength of the color filters are made substantially the same, color variations between a reflective state and a transmissive state are minimized. Here, when such a light source that emits light with a line spectrum is used as the light source, color mixing in RGB pixels can be reduced. By using a light source that emits a bright-line spectrum so that the peak wavelengths of light emission of R, G, and B go through only the transmission wavelength range of the corresponding color filters, the color purity is improved.
It should be noted that in the case of using a substrate made of a synthetic resin with a thickness of as thin as about 0.1 to 0.4 mm, even if the color filter layer is formed on an outer surface of the substrate, parallax is small and thus visibility is not reduced.
A light guiding plate for irradiating the liquid crystal layer with light projected from the light source preferably has a configuration in which the light is emitted only towards the electrode for transmissive display and not towards a region other than that. For example, the light guiding plate is such that V-shaped or sawtooth-shaped slots for emitting light are provided in regions corresponding to the electrodes for transmissive display and regions other than such regions are flat so that total reflection of light occurs on the internal surface thereof. Using a substrate made of a synthetic resin facilitates the processing of slots such as those described above.
Moreover, for carrying out a color time division driving, it is preferable that the response time of the liquid crystal layer be great, such as a few milliseconds. For example, an OCB mode, a ferroelectric liquid crystal mode, an antiferroelectric liquid crystal mode or the like is utilized.